Pyrimidine synthesis: carbamoyl phosphate synthetase II: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Pyrimidine synthesis: carbamoyl phosphate synthetase II

Sample item

A student compares the two carbamoyl phosphate synthetases. Which statement correctly describes carbamoyl phosphate synthetase II?

AIt is cytosolic, uses ammonia and is activated by arginineBIt is mitochondrial and is the rate-limiting enzyme of the urea cycleCIt is mitochondrial, uses free ammonia and requires N-acetylglutamateDIt is cytosolic, uses glutamine as the nitrogen donor and is inhibited by UTP

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. It is cytosolic, uses glutamine as the nitrogen donor and is inhibited by UTP

CPS II is part of the cytosolic multifunctional CAD protein in pyrimidine synthesis. It uses glutamine as the amino donor, is activated by PRPP and inhibited by UTP (feedback), and does not need N-acetylglutamate. CPS I is the mitochondrial urea cycle enzyme that uses ammonia and is activated by N-acetylglutamate.

Option by option

A. It is cytosolic, uses ammonia and is activated by arginine
Arginine activates N-acetylglutamate synthase in the urea cycle and does not regulate CPS II.
B. It is mitochondrial and is the rate-limiting enzyme of the urea cycle
The mitochondrial rate-limiting enzyme of the urea cycle is CPS I, not CPS II.
C. It is mitochondrial, uses free ammonia and requires N-acetylglutamate
This describes CPS I, the urea cycle enzyme.
D. It is cytosolic, uses glutamine as the nitrogen donor and is inhibited by UTP (correct)
Cytosolic location, glutamine donor and UTP inhibition define CPS II.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, It is cytosolic, uses glutamine as the nitrogen donor and is inhibited by UTP. CPS II is part of the cytosolic multifunctional CAD protein in pyrimidine synthesis.

Why is "It is cytosolic, uses ammonia and is activated by arginine" not the answer?

Arginine activates N-acetylglutamate synthase in the urea cycle and does not regulate CPS II.

Why is "It is mitochondrial and is the rate-limiting enzyme of the urea cycle" not the answer?

The mitochondrial rate-limiting enzyme of the urea cycle is CPS I, not CPS II.

Why is "It is mitochondrial, uses free ammonia and requires N-acetylglutamate" not the answer?

This describes CPS I, the urea cycle enzyme.

What concept does this question test?

It tests Pyrimidine synthesis: carbamoyl phosphate synthetase II in Biochemistry. It uses glutamine as the amino donor, is activated by PRPP and inhibited by UTP (feedback), and does not need N-acetylglutamate. CPS I is the mitochondrial urea cycle enzyme that uses ammonia and is activated by N-acetylglutamate.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Pyrimidine synthesis: carbamoyl phosphate synthetase II?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.